First off even in a war assuming no alliance and no true runes. Even the weakest country still has a chance of winning(or at least not being destroyed/anexed in the process). A good strategist know how to fight an overwhelming force and still remain victorious(or at least not lose) Don't take the island nations too lightly they may be small but there more than capable of winning a fight (I can't say just yet why I believe the island nations is strong becouse of the ban)
If i was zexen or the grassland I wouldn't start fighting one another knowing harmonia would be breathing down our necks.(this isnt an alliance mind you just a strategy)
About Harmonia are you really sure your rooting for the right country? I mean Shu was able to cause civil war there when higheast was invaded. And im certain that a good strategist would use the same tactic again against them. Another thing about them is the whole 3 class system if i was harmonia enemy(like Dunan for example) id try to make a secret alliance with third class citizens telling them that if we beat Harmonia they be free or something along that line. Harmonia will surely have a hard time with something like that
A revolution from below, a very Communist idea. Perhaps it could work, even if it was to be unsucessful and quashed, the Harmonian troops needed for internal affairs would weaken their ability to fight in the front lines, however just because Harmonia has had plenty of civil wars, it doesn't mean that its the only country that is potentially a breeding ground for that type of strategic tactic.
Dunan and Island Nations are both collective bodies of different nations, and can be subject to internal squabbles, so a good tactician could take advantage of this and create disunity, and a potential for a civil war is there.
The Grassland Clans would probably have to make some could of alliance since they would have attacks on 3 fronts directly from Zexen, Harmonia and Tinto.
I believe an alliance with Dunan would be the best idea since they could cut through the secret passage in Greenhill and even reclaim Tinto if sucessful, ousting Gustav in the process. Marlowe would make a good interim leader until a new leader was appointed since he pretty much has respect for Dunan due to Riou, and planned to write a novel about him, as well as the fact he's not as warlike as his master Gustav, he's a lover not a fighter.
Toran would probably be in an alliance with Dunan against Harmonia, since although they had problems in the past, Riou and Tir's heroic acts helped to cement the alliance Lepant made with Dunan on behalf of Toran. Also Harmonia would probably be wanting to reclaim their former capital and if there was to be a continental war, I would probably say that Toran would be first on Harmonia's hate list.
Zexen would either group together to fight against Harmonia, or fight against Tinto since they're the most likely to be attacked first since although Harmonia is also disliked, it is too powerful to attack initially and power must first be consolidated.
And now these three remain: faith, hope and love. But the greatest of these is love.
(I can't say just yet why I believe the island nations is strong becouse of the ban)